The defects and errors that led to the world’s deadliest garment-industry accident include the swampy ground the Rana Plaza was built on, the “extremely poor quality” construction materials and the massive, vibrating equipment operating when the eight-story building collapsed, a committee appointed by Bangladesh‘s government has concluded.

The committee recommended life prison sentences for the owners of the building and the five garment factories that operated there, though the charges they currently face carry a maximum seven-year term. Their report, submitted to the government on Wednesday, says nothing about the role an inadequate regulatory system played in the 24 April collapse, which left more than 1,100 people dead.

The disaster highlighted the hazardous working conditions in Bangladesh’s £13bn garment industry and the lack of safety for millions of workers who are paid as little as £25 a month. The 1,127 killed at Rana Plaza in the Dhaka suburb of Savar are among at least 1,800 Bangladesh garment-industry workers killed in fires or building collapses since 2005.

The investigating committee, appointed by the interior ministry, found that the ground Rana Plaza was built on was unfit for a multi-story building. “A portion of the building was constructed on land which had been a body of water before and was filled with rubbish,” committee head Khandker Mainuddin Ahmed said. He said the land had been swampy with shallow water.

“There were a series of irregularities.” The report found Rana had permission to build a six-storey structure and added two floors illegally so he could rent them out to garment factories. Past statements from authorities said the owner had permission for a five-storey structure and added three floors illegally.

The report also said the building was not built for industrial use, and the weight of the heavy garment factory machinery and their vibrations contributed to the building collapse.

Some elements of the report, including problems with building materials and heavy equipment, were previously mentioned by investigators.

Rana Plaza was shut down briefly after workers spotted cracks in its walls and pillars a day before the collapse. But the garment factory workers were called back to work, many of them forcibly, hours before the building fell.

The committee recommended that Rana and the owners of the garment factories be sentenced to life in jail if they are found guilty of violating building codes. Rana, three engineers and four factory owners have been arrested, but the building-code charges they face carry a maximum sentence of seven years behind bars.

The committee urged the government to ensure all those injured at Rana Plaza receive free medical treatment. More than 2,500 people were rescued shortly after the disaster.

Labour activists are among those who have blamed not just the owners but the government for the disaster. Government inspections of garment factories are infrequent and easily subverted by corruption, and the garment industry, by far Bangladesh’s biggest exporter, is highly influential in government.